Mixed Environment SharePoint

Great articles about mixed environment

Demystifying SharePoint server licensing

https://vigneshsharepointthoughts.com/2016/06/02/demystifying-sharepoint-server-licensing/

Mixing SharePoint Standard CALs and Enterprise CALs on the same farm (Old but valid)

http://blogs.msdn.com/b/bedwards/archive/2011/04/23/mixing-sharepoint-standard-cals-and-enterprise-cals-on-the-same-farm.aspx